The present invention relates to a soil reinforcing resin net and its production method, and also relates to a drawn product of polyethylene based resin which may preferably be used as a soil reinforcing net particularly suitable for preventing slip of filling-up soil or reinforcing soft soil.
In recent years, there is a tendency of filling-up marshy area or low level area for developing residential, industrial or roadway sites. In such site development, since buildings are constructed on the filling-up soil, it is required to prevent collapse of the filling-up soil. To meet such a requirement, filling-up of soil for developing residential site or the like has been made by repeating the step of laying synthetic resin-made reinforcing nets on underlying soil at specific intervals and banking soil thereon to a thickness of several tens cm. Specifically, in recent years, to meet strong demands to increase construction of dwelling houses, there is a tendency to prepare residential site on filling-up soil or soft soil. In this case, the slip of such residential site must be perfectly prevented. In general, to prevent the slip of site and stabilize the filling-up soil, there has been adopted a method of arranging layered nets in soil. The net is generally made from polyethylene which is subjected to uniaxial or biaxial drawing for realizing molecular orientation thereby increasing the strength of polyethylene.
The above soil reinforcing net has a large number of holes. When the net is laid in soil, upper and lower filling-up soil layers arranged with the net put therebetween are jointed to each other through the holes of the net, to prevent collapse the filling-up soil. To effectively prevent collapse of filling-up soil, the net is formed of a sheet of a resin having a high drawability such as high density polyethylene, which sheet is drawn uniaxially or biaxially for making the molecular orientation linear. To be more specific, the sheet is drawn in such a manner that the molecular orientation of the sheet is matched with the direction in which collapse of filling-up soil is anticipated to occur, to prevent elongation of the net when a slip force is applied from filling-up soil to the net, thereby keeping the strength of the net.
FIG. 13 is a schematic view of such a reinforcing resin net 30. The resin net 30 is produced by punching a polyethylene sheet 20 shown in FIG. 14 to form round or rectangular holes 21 therein, and drawing the sheet 20 in a uniaxial direction. By drawing the sheet 20, each hole 21 becomes an elongated hole 31 having a pair of opposed ribs 31a and a pair of opposed bars 31b. In FIG. 13, reference numeral 40 designates a non-punched portion; 40a is a drawn rib portion: and 40b is a non-drawn bar portion.
The resistance force of the net of this type for preventing collapse of filling-up soil is generally tested by laying soil below and above the net 30 to specific thicknesses, pulling the net 30 out of the soil, and taking the pull-out resistance as the resistance force of the net. Such a pull-out resistance has been substantially determined by a thickness of the bars 31b extending in the direction transverse to the pulling direction, that is, the area against the pulling direction. Accordingly, to increase the resistance force of the net 30, there has been taken a countermeasure of increasing the thickness of the sheet 20 as a base of the net 30 or making short the vertical gap, that is, the amount of soil between the two adjacently laid nets 30. The increased thickness of the sheet 20, however, is inconvenient for raising the construction cost. Further, since the boundary between the non-drawn portion and the drawn portion forms a smooth slope, even if the thickness of the non-drawn portion is increased, it is difficult to ensure a desired pull-out resistance.
An object of the present invention is to provide a soil reinforcing resin net capable of improving pull-out resistance in soil, and its production method.
Another object of the present invention is to provide a drawn product of polyethylene based resin with improved creep characteristic and increased strength, which is suitable for soil reinforcement.
According to a first aspect of the present invention, there is provided a soil reinforcing resin net including hole portions arranged in the longitudinal and transverse directions of the resin net, each of which is formed by a pair of opposed ribs and a pair of opposed bars, wherein projecting portions formed typically into fillet shapes are fixed on bar portions, preferably, in the direction transverse to the drawing plane of the hole portions. The projecting portions may be fixed on the bar portions by vibrational fusion, ultrasonic fusion or hot plate fusion.
According to a second aspect of the present invention, there is provided a method of producing a soil reinforcing resin net, including the steps of: punching a resin sheet to form hole portions arranged in the longitudinal and transverse directions of the resin sheet; drawing the resin sheet having the hole portions in the uniaxial direction, to form elongated hole portions each having a pair of opposed ribs and a pair of opposed bars; and fixing projecting portions on bar portions by vibrational fusion, ultrasonic fusion or hot plate fusion. In this method, the projecting portions may be fixed on the bar portions in the direction transverse to the drawing direction of the hole portions.
According to the soil reinforcing resin net of the present invention, since the projecting portions, for example, fillet portions are fixed on the bar portions extending in the direction transverse to the drawing direction of the net, it is possible to significantly increase the pull-out resistance in soil of the resin net.
The soil reinforcing resin net is generally made of a high corrosion resisting polypropylene or polyethylene sheet which is little compatible with adhesive, and therefore, it has been difficult to fix separate projecting portions to bar portions of the sheet by adhesive joint.
According to the present invention, however, the bar portion is fused by vibration, ultrasonic waves, or hot plate, and the projecting portions are fixed on the bar portion thus fused. To be more specific, the projecting portions are brought into press-contact with the bar portions, and are fixed thereto by the above-described fusing means such as vibration. In other words, according to the present invention, the projecting portions can be fixed to the bar portions not by using the chemical adhesive bonding mechanism but by using the mechanical fixing mechanism. For example, the fixing of the projecting portions to the bar portions of the resin net by vibrational fusion is performed by bringing the projecting portions into press-contact with the bar portions of the resin net, applying fine vibration to the press-contact portions to give an energy to the bar portions, being press-contact with the projecting portions, of the resin net thereby softening them, and re-hardening the softened bar portions of the resin net in a state in which the softened bar portions surround the projecting portions. Additionally, the projecting portions can be fixed to the bar portions of the net in the state before drawing.
The projecting portion is mainly made from the same material as that of the resin net, such as polypropylene or polyethylene. The shape of the projecting portion is not particularly limited and, for example, it may be a columnar, three-dimensional or groove shape. The length of the projecting portion can be also arbitrarily selected.
According to a third aspect of the present invention, there is provided a method of producing a soil reinforcing resin net, including the steps of: punching a resin sheet to form hole portions arranged in the longitudinal and transverse directions of the resin sheet and simultaneously forming, by pressing, a plurality of projections on bar portions between the hole portions at specific intervals in the transverse direction; and drawing the resin sheet having the hole portions in the longitudinal direction to form elongated hole portions each having a pair of opposed ribs and a pair of opposed bars, thereby forming the projections on the bar portions of the drawn sheet.
According to the third aspect of the present invention, since a plurality of the projections are formed by pressing, before drawing, on the bar portions of the resin sheet, which become the bar portions extending in the direction transverse to the drawing direction of the resin net, it is possible to significantly increase the pull-out resistance in soil of the resin net.
The soil reinforcing resin net is generally formed of a high corrosion resisting polypropylene or polyethylene sheet which is little compatible with adhesive, and therefore, it has been difficult to fix separate projecting portions to bar portions of the sheet by adhesive joint.
According to the present invention, however, the projections are formed by pressing, before drawing, on the bar portions of the sheet, which become the bar portions extending in the direction transverse to the drawing direction of the net. The projection may be formed at a crossing portion surrounded by four of the hole portions in the non-punched portion; however, it may be formed at the bar portion (non-punched portion) put between the hole portions arranged in the longitudinal direction of the resin sheet. With this configuration, upon formation of the net, the projection is formed into the shape functioning as a tire spike between the elongated hole portions. Accordingly, the resin net exhibits the spike effect to fill-up soil. From the viewpoint of the production steps and production cost, it may be desirable to form the projections by pressing simultaneously with punching of the hole portions.
Additionally, in the case of forming each projection at a crossing portion surrounded by four of the hole portions in the non-punched portion by pressing, stress concentration occurs at the projection upon drawing the sheet, and accordingly, it is difficult to keep the original shape of the projection after drawing of the sheet.
According to a fourth aspect of the present invention, there is provided a method of producing a soil reinforcing resin net, including the steps of: punching a corrosion resisting resin sheet generally made from polypropylene or polyethylene to form hole portions regularly arranged in the longitudinal and transverse directions; drawing the resin sheet having the hole portions in the uniaxial direction to form elongated hole portions each having a pair of opposed ribs and a pair of opposed bars; and forming projections on bar portions of the drawn sheet by pressing. The projections are preferably formed on the bar portions between the elongated hole portions.
In this way, the resin sheet is punched to form the hole portions regularly arranged in the longitudinal and transverse directions and is drawn in the uniaxial direction to form the elongated hole portions. The projections are then formed on the bar portions remaining in the direction transverse to the drawing direction by elastically deforming the positions, corresponding to the projections, of the bar portions by pressing using a press on which upper and lower dies having recesses and projections respectively are mounted. According to this method, the projections can be formed into predetermined shapes, and the shapes of the projections are not deformed because the projections are formed after the drawing step. As a result, the resin net of the present invention can certainly ensure an improved pull-out resistance in soil.
The shape of the projection is not particularly limited; however, it is generally formed into a cylindrical shape. To be more specific, the cylindrical projection is formed on the front side of the net and the recess corresponding to the projection is formed on the back side of the net in accordance with the shape of the cavity of the die used in pressing.
According to the present invention, there is also provided a drawn product of a polyethylene based resin obtained by drawing a polyethylene based resin composed of a linear ethylene main chain having 0.3 or more (in average) of saturated hydrocarbon groups branched thereto in one molecule.
It is known that when high density polyethylene is drawn, ethylene molecules are oriented in the drawing direction, whereby a drawn product having a high molecular orientation is obtained. The drawn product of high density polyethylene is obtained by zone drawing, roll drawing, high pressure extrusion, high frequency hot drawing, gel drawing, or melting crystallization, and is expected as a material having a high strength and a high elastic modulus. The drawn product of high density polyethylene having a high strength and a high elastic modulus, however, has a disadvantage that the creep characteristic as a criterion to evaluate the shape stability in long-term use. If the drawn product of high density polyethylene, which is poor in creep characteristic, is used for a soil reinforcing net, there may occur a problem. According to the drawn product of the present invention, it is possible to solve such a problem.
In general, linear high density polyethylene having a density of 0.94 g/cm3 or more is different from low density polyethylene having a density less than 0.94 g/cm3 and many branched chains, and thereby being soft. In the case of drawing high density polyethylene, it is easy to be crystallized because of its linear main chain so that the structure of the drawn product is composed large crystal portions each having a lamellar structure in which the molecular chain is arranged in parallel to the drawing axis and slight amorphous portions each being put between the crystal portions. Since the crystal portions are rigid, the creep characteristic is dependent on the stretching state and entanglement of molecules in the amorphous portions, and since the branched chains slightly introduced in the linear main chain are present in the amorphous portions, the entanglement and the stretching state of molecules in the amorphous portion are increased due to the presence of the branched chains. In this way, the present inventors have found that when a drawn product of high density polyethylene based resin in which branched chains are introduced in the linear main chain is subjected to a creep test in which a load is applied to the drawn product for a long time, the elongation of the drawn product is suppressed at minimum, and that when the drawn product of the polyethylene based resin excellent in creep characteristic is used for reinforcing filling-up soil or soft soil, it is possible to obtain a desirable soil reinforcing effect stably kept for a long term.